1. Field of the Invention
This invention relates to an antivibration rubber device such as a suspension mount rubber and an engine mount rubber of an automobile vehicle, and more particularly to an advantageous mechanism of an orifice plate and an elastic block provided within a fluid chamber to mainly isolate medium to high frequency vibrations.
2. Description of the Prior Art
One example of such an antivibration rubber device is disclosed in Japanese Laid-Open Patent Publication (Kokai) No. Sho 64-1566 comprising a first bracket secured to an automobile vehicle body, a second bracket secured to a vibrating member, a rubber portion disposed between the first and second brackets, a fluid chamber provided within the rubber portion, and an elastic partition wall dividing the fluid chamber into two chambers. The partition wall is provided with an orifice passage adapted to communicate with the two fluid chambers and an orifice plate provided within at least one of the divided fluid chambers to perform a relative movement integrally with a connecting member, thereby mainly isolating the medium to high frequency vibrations.
Since this orifice plate is formed to have a generally circular shape in the principal input direction Z of the vibrations to be isolated, in each vibration direction of the two directions X and Y which are perpendicular to the principal vibration input direction Z and intersect at right angles to each other, the projection surface shape and the projection area are almost equal. Therefore the minimum value of the dynamic spring constant in each vibration direction of X and Y is also almost the same level and the resonance frequencies a and b in each vibration direction of X and Y are also substantially equal as shown in FIG. 10(A).
In the figure, a horizontal axis shows the frequency of the input vibration and a vertical axis shows the dynamic spring constant, in which the letter c shows the resonance frequency in the direction Z. In the following description, the direction represented by X, Y, and Z means the direction of vibrations input along any of the axes in the three directions X, Y, and Z which intersect at right angles to each other.
However, in a certain specification, the resonance frequency in each vibration direction of X and Y is not made equal. For example, the resonance frequency in the direction X is the same as before, but it may be required to be capable of performing the displacement control, while in the direction Y, it may be required to move the resonance frequency to a higher frequency side. However, in the prior art, it has not been possible to change a characteristic every different vibration direction such as the directions X and Y.
On the other hand, as one example of an elastic block, there has been proposed an engine mount in which an upper bracket on the side of an engine and a lower bracket on the side of a vehicle body are connected by the elastic block made of rubber which is formed as a cylindrical member of a generally truncated cone shape when viewed from a side. There is also a specification in which a spring constant in the principal input direction Z of the vibrations to be isolat and, the ratio of the spring constant of each of the two directions X and Y which are perpendicular to the input direction Z (and also intersect at right angles to each other) is required to be more than 1. In this case, the elastic block is formed as a cylindrical member of a generally truncated cone shape when viewed from a side, and the open end on the side of a smaller diameter which is the upper portion of the elastic block is formed with a taper surface slanting inward, wherein the upper bracket formed with the corresponding taper surface is arranged to contact the taper surface of the elastic block.
With such a construction, it is possible to realize the ratio described above, but since the elastic block is principally loaded with load in the shearing direction, the creep amount due to the engine heat increases and it has the drawback that the elastic block can not be used when the allotted load is over 100 Kilograms (kgs). In this case, it has been proposed to use a stiffening ring at the middle portion of the elastic block, but it has the drawback of making the products large-sized.
It is therefore an object of the present invention to provide an improved antivibration rubber device which can overcome all of such drawbacks as found in the prior art.